TYGA Street NX-5 RS250R-S

This is a TYGA project with a twist. Historically, we have always begun with a road bike and we then turned it into either race replica or a track bike. In this instance, we began with a GP bike and we turned it into a road bike. You may be wondering why, and we’d struggle to give you a rational answer other than to say why not?

I think most 250 sportsbike owners have thought about or discussed building the lightest most powerful quarter litre bike for the road and how much fun it would be. So we’ve just run with that idea and instead of starting with a street bike and stripping it down to the bare essentials we’ve gone the other way, started with a completely stripped down GP bike and added as little as possible to make it street compliant. We like challenges! It sounds simple in theory but it required a bit of lateral thinking. Without further ado, we present you with the RS250R-S

So let’s start with the basics. There was never going to be any doubt about which GP bike we would modify. We have always had an affinity for the Honda RS250 NX5s and we have a strong admiration for HRC. We know these bikes are now getting very collectable and some people might wonder if it is a wise to modify one. Well, the 1993 bike used for this project, while being a clean and tidy example, was not pristine and was missing stock bodywork and other components so was a worthy candidate to be taken in a different direction than just a straight restoration. So all you purists out there reading this can relax. No show quality GP bikes were harmed during the making of this project!

Anyway, now we’ve got that out the way, a stock NX5 measures high 70s horsepower. That doesn’t sound that impressive until you consider the weight, or lack of, and that the power is measured at the back wheel not on an advertising spec sheet. It only weighs 98 kg dry which is the same as my 110cc scooter! The challenge was to keep as close as possible to these numbers while making the necessary changes to make it into a street bike.

Many of the things which will make this bike street compliant involve wiring. Early on in the project, we decided to go for a separate harness in tandem with the stock NX5; one to run lights, horn etc. off. Matt is a very competent auto electrician, but he didn’t want to risk overlooking something, end up having a spike in the voltage and damaging any difficult to find and expensive NX5 electrical components. Besides, the concept is that this bike can be ridden to the track as a street bike and quickly converted back to pure racer so in this sense the extra removable loom makes sense.

What about the charging system? Yes, well, we thought about fitting a lighting generator but, with only LED lighting and the anticipated relatively short trips this bike will make, the battery can easily hold enough juice. Look at it another way, you will need to strip down the top end for a check or rebuild by the time the battery needs a full charge. (This isn’t supposed to be a daily ride but a Sunday morning canyon racer!)

So what exactly does the extra harness do? Well, we have a full lighting system including hi/lo headlights, taillight with brake light and turn signals. In addition to the lights, there is horn hidden behind the radiator and that is pretty much it. The harness is linked up to a lightweight lithium battery which in turn is held in place by a bespoke light aluminium battery box which sits under the seat and hooks on the subframe nice and discreetly. Power to the street harness is switched on via a secret lighting switch hidden away to prevent anyone fiddling with the lights when unattended. The only other thing to say about the wiring is that, because the main harness for powering the engine is independent, the ignition to the engine is controlled by the stock run switch on the handlebar. There is no ignition key and really it doesn’t need one because I am quite sure this bike is not going to be left unattended in any public space.

Apart from the wiring, there were some other changes which the bike required for the street, at least to make it somewhat practical. Firstly, the NX5 usually runs open carbs. The later model has ram air and an enclosed air box. Our early model had just a tray so everything from stones to flies to passing bats when riding at night might get sucked into the carbs. This would play havoc with the jetting and wear out the single ring pistons at an even more alarming rate than it does anyway. The solution was to install one of the later ram air model enclosed air boxes which TYGA makes, and open the inspection door at the front to allow for air to enter the forward section. Within the air box itself, is a modification of the stone guard. Instead, of the guard, we installed a sheet of air filter supplied by Kitaco and then cut to the correct shape to fully seal the rear part of the air box where the carbs suck. We also made reliefs in the filter foam to allow the throttle and choke cables to pass through it, which brings me to the next modification.

On a stock NX5, the chokes are operated by two knobs operated on the carb bodies themselves which can be accessed by either lifting the fuel tank and grabbing hold of them from the top or through the side hole in the air box. While this works ok, in fact some old school racer types might even feel this is part of the ritual of starting a 2 stroke GP bike ready for the grid, it is a somewhat clumsy and unsophisticated method for a street bike. Most bikes these days don’t even need a choke. Ok, as the rider, you might know that you are just working the choke levers to get the bike started, but with a fuel tank half off your bike and the engine spluttering, the two stroke sceptics at your local café looking on are going to be grinning ear to ear and making jokes about unreliable two strokes. We can’t be having that! So the solution is an elegant and discrete choke lever near the radiator which can be operated nonchalantly and discretely without making a spectacle of yourself. This, like most things, is a lot easier said than done but the design once we sorted it out is actually quite simple and easy to make. Our 3D printer came in handy here and the chokes are now operated via cables and the hardware in the photos to allow them to pull them up and allow them to return. The lever is modelled on a Honda NSR250 one but is a ‘mini me’ version. The cable splitter is a shorter version of the NX5 throttle cable splitter.

It works flawlessly and because most components of this conversion are made from ABS plastic, does not add much to the weight of the bike, which is an all important consideration.

The NX5 is designed to be a pure race bike, and as such, it doesn’t need a sidestand. It doesn’t even have an obvious place to mount one to. Being a pro arm, it is not even like you can easily mount one of those aftermarket MotoX style sidestands either. So this issue required a bit of lateral thinking. Leaning this gem of a bike up against a wall would be precarious and ugly. As mentioned earlier, our intention was to make every modification temporary so that it can be reversed at some time in the future or removed to go racing. An easy solution would have been welding or bolting on sidestand mounts but that would be a cop out. We needed to find a more elegant solution. As anyone who has worked or even seen one of these bikes up close will know, there is not exactly a lot of spare places to put things and after quite a lot of discussions and back and forth on solutions, we settled on the swing arm pivot to mount a bespoke sidestand.

The sidestand pivot is held on the left side by means of a spring which goes through the swing arm shaft and prevents the sidestand pivot from falling out. The swing arm pivot bolt has a castle nut to lock the adjuster into position and this is comes in handy to also lock the sidestand pivot into position and prevent it from rotating. Of course not every castle nut will be in the same orientation so the lock stop on the sidestand pivot can be located in one of a number of orientations to enable it to rotate to the desired amount and no further so that the bike sits gracefully at the correct angle and is fully supported. After deployment, the sidestand is rotated in the normal way, but continues upwards and forwards until it fits neatly away between the frame and side of the fairing in a custom made cup which is linked to the dash sidestand warning light. It does require temporary detaching of the side fairing and R clip but this is an easy and quick job. We were initially concerned that the sidestand would look cumbersome and detract from the looks of the bike but we needn’t have worried. Hidden away behind the fairing, you hardly notice it at all. Of course, if it gets annoying and we don’t need it for track use, one spring and the whole assembly can be easily removed. Weight, by the way, is much less than say a stock NSR250 sidestand and comes in at just under 600g.

Funny as it may sound, we nearly forgot to install a speedometer. So, a quick trip to the local cycle shop and we sourced a wireless bicycle speedo. We also sourced some miniature magnets to replace the ugly one supplied with the speedo kit and Matt 3D printed a custom magnet holder which is velcroed on the wheel rim. The sensor itself is hidden behind the front fender on a special custom built bracket. The meter itself was incorporated into the dash. More on that later.

One thing we did consider, but up to now we haven’t yet accomplished, is a way of starting the engine without a push start. I must say, this is one aspect in which we are still not completely satisfied. Options would be fitting a starter motor but this would require quite a lot of additional weight and use precious battery power so then we would need a generator or a bigger battery which means even more weight. A kick start is not really viable without major modifications to the engine which is too precious to mess with. We are open to ideas but apart from parking on top of a hill and casually and silently gliding downhill out of sight from those pesky 2 stroke sceptics at the café, then unfortunately the actual starting ritual is going to be an old school bump start.

As well as the road going technical modifications, our intention was to give this project a contemporary design and to provide it with the best in high performance. NX5s come equipped with very impressive brakes from HRC but we took this opportunity to upgrade nevertheless. The RSW250 Nissin forks are World GP250 championship spec and were sourced from Spain. As you’d imagine, they are exquisite. Stiction is almost non-existent and the bottom radial mounts are made from magnesium.

The bottom triple is also RSW250, but to match the NX5 geometry and provide the correct ride height, a bespoke TYGA top triple clamp was specially made allowing for more ride height. While we were at it, handlebar clamps were made to suit the forks and mated to our handlebar tubes. The RSW250 specific Brembo rotors were donated by Harc-Pro. Calipers are nickel plated Brembo GP4-RXs and these are linked to the Brembo 19RCS master cylinder via HEL brake lines to complete a very high spec braking system.

Needless to say on a 100kg bike on the street, the brakes are more than sufficient. The rear shock is Ohlins, but other than that, the other chassis modifications are mostly in detail such as extensive use of titanium bolts and fasteners etc. Wheels are exclusively supplied to TYGA to our specification to suit the NX5 with a 3.50 front and 5.50 rear made from forged aluminium.

Weight is on a par with the HRC magnesium hoops but as well as looking more modern, they inspire more confidence than trusting possible cracking of the 25 year old magnesium Magteks. Other TYGA components are the stainless steel exhausts with carbon/Kevlar silencers, CNC step kit, 3D printed fuel filler cap and various stays and brackets such as the front and rear brake reservoir holders.

Bodywork is by TYGA Performance and made from carbon with a Kevlar layer on the inside. The seat cowling is an adaptation of the TYGA Performance RGV 250 (BPFT-9026) unit but unlike the other NX5s we have installed this seat cowling to, we retained the light unit and completed the street conversion at the rear with a carbon registration holder and turn signals The lower fairing is modelled from an RSW250 design, as are the sides of the upper cowling. The front section of the upper is moulded from our NSR250 GP-T and grafted to fit the RS250.

The fairing is attached to the frame by custom made stays with quick release R clips.

The TYGA GP-T headlight required a custom meter stay to prevent collision with the RC valve servo motor, a consideration HRC did not have to worry about! While we are at the front end, the instrument dash, as mentioned earlier, is 3D printed from ABS and incorporates the stock HRC temperature gauge repositioned higher for better visibility and the bicycle speedometer. In the centre is the stock rev counter and idiot lights for the headlight indicator, turn signal and sidestand. To provide illumination at night, instead of tampering with the instruments to provide back lighting, we took another approach and installed front lighting provided by a custom made LED array attached to the front of the headstock.

Both the speedometer and temperature gauges have carbon surrounds to retain the feel of the HRC racer. The front fender is a TYGA RC211V style model designed for an NC35 which has then been adapted to fit the RSW250 front end. The paintwork is done in our TYGA Performance corporate colours with the orange being the same fluorescent colour found on our various other projects so what with the projector beams and bright paint, this bike will be noticed approaching other road users.

That just about sums up the specification and now it is time to look at the performance. For the lucky few who have experienced sitting on a street 250 sportsbike then if you imagine how an RGV, Aprilia RS-250 or even an NSR250 feels, well the NX5 is familiar in some ways but different in others. The GP bike by comparison is much smaller and lighter. It is fair to say that the RS250 was never designed to fit a full grown adult of western proportions, and no concession is made whatsoever for comfort. In comparison to say an NSR250 which is hardly lardy or soft, everything feels smaller, stiffer, harder and lighter. You know this bike is designed for one purpose and one purpose only and that is to race. Hmmm, so how is this going to feel on the street?

Only one way to find out, sidestand up, chokes on and now for that embarrassing bump start. The engine quickly coughs into life and all your senses are given a treat. There is clatter of the dry clutch, the induction roar, the howl of the exhausts and that is just the sense of sound. On top of that, the premix burns inefficiently on choke and clouds of blue smoke waft through the air. With the breeze, the bike becomes engulfed and with the beams of the projector beam headlights shining through, it gives you a sense of drama similar to a new model launch; only this smoke comes with the associated aroma of two stroke oil burning not dry ice. It gets in your nose and you can taste the A747. Being a race bike, the engine needs to be under load so you need to constantly blip the throttle in neutral. As the revs rise each time you blip, it vibrates a little. It is not a nasty vibration like on an old 70s two stroke but just enough to let you know that this is a proper racer, no sissy rubber engine mounted NSR250 engine here!

In no time, the engine is warm enough to disengage the chokes and you need to continue to blip the throttle with more revs to get the bike up to temperature. Once 55 degrees is showing on the HRC digital display, it is time to engage the dry clutch and get underway. Gear ratios are close so getting away requires a fair amount of slipping the clutch or you risk stalling it. Once the clutch is out and you are ready to change up, you need to remember it is a race shift but once you have got the hang of it, the RS250R-S is surprisingly easy bike to ride in traffic on the highway.

The NX5 is much more tractable than say an NF5 and the jetting is right on the money with smooth delivery from 4,000 rpm. The overall small size, geometry and light weight allow for easy filtering through the traffic. The brakes and suspension are so unfazed by the everyday street conditions that it helps to calm an otherwise anxious first ride. I mean, even though you know it has all the right street gear, it still feels a bit naughty to be out on a public highway on such a bike. After a brief ride, I flick on the right indicator and move to the slip road to make a U turn. With some difficulty due to the limited steering lock, I manage to turn it in the two lanes on the other side of the road. Of course, with such a bike, I was getting looks before, but now I’m feeling a bit self-conscious due to more or less being on a collision course with a street vender on a scooter and sidecar going up the wrong way in the hard shoulder. Other U turn traffic is building up behind me and also on the highway to my left so I’ve pretty much blocked one entire carriageway and I feel I’m being stared at… I can’t turn tighter and I can’t back up because there is a car behind. Not much I can do except wait and hang out in the slow lane at a 45 degree angle to the kerb and let the lumbering street seller outfit pass by me. Having blocked the traffic I have given myself some clear road as a result, I give it some revs, let out the lever to easy the noisy clutch and pull away in first gear which feels like 3rd creating a cloud of smoke in the process. I concentrate on the race shift and the feeling is not so much of one of great power but one of nimbleness and lightweight and this is reflected in the rapid acceleration that eventually comes once I’ve got into the power band. Not wishing to push the top end performance quite yet, I short shift in the top two gears (it is geared for 270 km/h) and then soon slow down as my turn off to base is fast approaching. I apply the brakes with one finger lightly on the lever and I feel the bike pull up way too fast or I will stop in about half the distance I was expecting so I let the lever go and take another bite and come gently to a stop at the junction.

Back at base and time to reflect. My expectations were quite high but I’m really impressed. Not only with the performance and proof of concept but also that nothing broke, nothing came lose or needed adjusting. My only negative criticisms would be the cycle speedo lags and is really only useful at constant speeds but seeing as we nearly forgot to install it, I’m not going to worry about that small detail. Turning circle of a super tanker is going to be planned for in future too but not a biggie either while on the open road. Of course, we didn’t build this bike for commuting and the next step is to get out on the back twisty roads to push the limits. This sounds like a job for Matt!

Matt's Impressions:

There was plenty of excitement running though us all on the run up to the first real world shakedown of the RS250R-S. Would it be user friendly or a complete waste of our efforts?

Having ridden and raced 250GP bikes, I am quite aware of the (huge) differences between a full on GP bike and a road going race replica, such as our favourite Honda NSR250 for example. The GP bikes have a rigid chassis with firm suspension, whereas the street going replica have a somewhat more forgiving and generally less sophisticated setup.

First task is to pick the RS up off the fully CNC machined side stand and tuck it away into the faring. Out of sight, out of mind. Flicking on the electrics brings all the street legal gear into play. Still feels like a GP bike though.

The starting procedure is typically 90’s race protocol. Fuel on, choke on, select first gear and give it a push. However, now things are a little easier than the GP bike, as the fuel tap can be accessed without having to lift the tank or open little doors, and the remote choke lever makes things very simple. A quick push, drop the clutch and the naughty little stroker splutters into life.

Back into neutral, chokes off and gentle blipping of the throttle brings the temperature up to 55°C and into the “Go Zone”. First gear…..a fistful of revs and we’re away!

Blimey!! Need to have a twiddle with the suspension. A little less compression damping sorts out the choppiness of the race orientated forks and shock. Off we go again.

The engine is of course aimed more for high rpm use, so I didn’t expect much midrange. The final gear ratio is as low as we have, but this was never really meant to be used on the road. Oh well, just have to ride a gear lower than one normally would. But once on the move this little lightweight stroker really gets a move on. Even short shifting through the slick gearbox, things are happening fast.

Corner coming……hard on the brakes. Wow, these brakes are better than expected, with the RSW forks offering a firm but very stable feeling. The front Pirelli grips hard as we sail through the turn on rails. On with the gas and the rear tyre hooks up while the Ohlins shock keeps things well under control and we shoot off down the straight.

“Nimble” is a word that is often used by motorcycle journos, but it’s a word that should be reserved for use only with HRC’s finest. Absolutely no trouble at all flicking the bike back and forth through the twisties. A few little bumps here and there do try to upset things, but the HRC steering damper and top shelf suspension stops things getting out of control.

Potential top speed is a little limited by the low final gearing at around 135mph, but this is of course plenty fast enough for road use. Of course, we never go above legal speeds so won’t know the true top speed until we get it to the track.

So the pros and cons?

Well, the first “pro” is of course the pose value. A fully street prepared, 250cc, two stroke Grand Prix machine is hard to beat parked outside your favourite coffee shop/chip shop. And with its high power to weight ratio and outstanding agility, it’ll beat most street bikes on the back roads on a sunny Sunday morning. The top level brakes and trackday tyres will bring you to a safe stop should nature (or other road users) spring any surprises.

The “cons” are related to this being a GP bike. Having to push start is a bit of a pain, but as the jetting is pretty spot on it’ll fire into life easily and can be done straddling the bike and paddling along, so not a disaster. The turning circle is also rather limited, requiring a couple of goes at turning it around in a regular sized street. Comfort is somewhat compromised by the hard seat, but that’s not a big problem because if your bum is in the chair for too long then you’re on the wrong type of road! Fuel anxiety is another limiting factor. This is a full on, early 90s race spec engine and as such drinks premixed avgas 100LL and Castrol A747 at an alarming rate, so short trips are required. This is certainly no motorway cruiser! Of course we could modify it to run pump fuel, and then mix the oil in the gas station, but where’s the fun in that? And everyone loves the smell of avgas and A747 right?.....right?

So, in a nutshell, the TYGA RS250R-S is an amazing little street bike that’ll make most other bikes on the road look silly. Short trips through the winding back roads are where it excels, so forget using it as an everyday commuter.

What would I change? Not a thing! It’s perfect just as it is. Maybe my opinion will change in another hour or so when the adrenalin has worn off, but unlikely. The biggest problem now is what to build and ride next as the RS250R-S will be very hard to better.